Roots pumps, claw pumps, and double-screw pumps are known, but those machines include two shafts that are synchronized in rotation by gears that are lubricated and are therefore not entirely dry.
Spiral pumps referred to as "scroll pumps" are also known, but they are expensive because it is difficult to obtain the very accurate outline that is required for the spirals. Furthermore, they cannot pump condensates.
Dry vane pumps are also known, but the vanes wear quickly and give rise to considerably lower performance levels, and short pump life, and the vacuum chamber is polluted by the wear products. Diaphragm pumps are also known, but the diaphragms have a short life, and piston pumps are known, but they have low performance levels and high noise and vibration levels.
The invention relates to a new type of dry primary pump which enables most of the problems and drawbacks of known dry primary pumps to be overcome. The new type of pump is a positive-displacement machine having orbital motion and being hypertrochoidal in geometrical shape.
The machine comprises a cylindrical piston, a cylindrical casing surrounding the piston, and a crank shaft whose axes are parallel to those of the cylinders delimiting the shapes of the piston and of the casing, the crank shaft being in rotary relation with the piston and with the casing.
The term "cylindrical" is used herein in its broad mathematical sense; with neither the piston nor the casing necessarily being in the form of a right circular cylinder. In particular, in that machine, the cylinder defining the shape of the piston has an order of symmetry about its axis equal to S.sub.p, whereas the cylinder of the casing has an order of symmetry equal to S.sub.c ; with S.sub.p and S.sub.c being chosen so that they differ from each other by unity. Furthermore, the geometrical shapes of the piston and of the casing are chosen so that the two elements correspond directly to each other.
One of the elements (i.e. the casing or the piston) has an outline P.sub.1 which corresponds to a curve uniformly distant from a closed hypertrochoid, having no crunodes and no cusps, excluding hypertrochoids that are degraded into hypotrochoids, epitrochoids, or peritrochoids. The outline P.sub.1 may also be at zero distance from such a hypertrochoid, and may therefore correspond thereto. Hypertrochoids are defined in French Patent 2,203,421. The other element has an outline P.sub.2 which is the envelope of P.sub.1 in relative orbital motion defined by two circles C.sub.1 and C.sub.2 having respective centers and radii (O.sub.1, R.sub.1) and (O.sub.2, R.sub.2), the circles being respectively secured to the outlines P.sub.1 and P.sub.2, and rolling on each other without slip via internal contact, .vertline.O.sub.1 O.sub.2 .vertline. indicating the distance E between the axes of the crank shaft.
Machines satisfying those characteristics may be grouped into four families depending on the nature of the element whose shape is defined by P.sub.1, and depending on the comparative values of the radii R.sub.1 and R.sub.2. The following should be distinguished:
machines for which P.sub.1 is the outline of the piston and P.sub.2 is the outline of the casing, which outline corresponds to the outer envelope of P.sub.1 in the orbital motion of P.sub.1 relative to P.sub.2, for which R.sub.1 =S.sub.p E and R.sub.2 =S.sub.c E=(S.sub.p +1)E (family I);
machines for which P.sub.1 is the outline of the piston and P.sub.2 is the outline of the casing, which outline corresponds to the outer envelope of P.sub.1 in the orbital motion of P.sub.1 relative to P.sub.2, for which R.sub.1 =S.sub.p E and R.sub.2 =S.sub.c E=(S.sub.p -1)E (family II);
machines for which P.sub.1 is the outline of the casing and P.sub.2 is the outline of the piston, which outline corresponds to the inner envelope of P.sub.1 in the orbital motion of P.sub.1 relative to P.sub.2, for which R.sub.2 =S.sub.p E and R.sub.1 =S.sub.c E=(S.sub.p -1)E where S.sub.p &gt;1 (family III); and
machines for which P.sub.1 is the outline of the casing and P.sub.2 is the outline of the piston, which outline corresponds to the inner envelope of P.sub.1 in the orbital motion of P.sub.1 relative to P.sub.2, for which R.sub.2 =S.sub.p E and R.sub.1 =S.sub.c E=(S.sub.p +1)E (family IV).
Other machines may be derived from machines belonging to any one of the four preceding families. An outline P.sub.2 may be used, having at least one portion corresponding to the envelope P.sub.1 in its motion relative to P.sub.2, and at least one portion outside the envelope in the case of families I or II, and inside the envelope in the case of families III or IV, the various portions connecting together to define a closed curve.
The outlines of the piston and of the casing of such a machine offer the advantage of being machinable by mass-production machines (lathe-type machines), and this reduces the cost of the piston and of the casing.
The orbital motion of such machines may be achieved, either by internal gearing having parallel axes, the gear wheels being respectively secured to the piston and to the casing, and having respective pitch radii that are equal to R.sub.1 and R.sub.2, or else if the geometrical shapes of those surfaces of the piston and of the casing which are in contact with each other enables sufficient throughput, and if the fluid conveyed by the machine is sufficiently lubricating, then the gearing may be omitted and the relative orbital motion is directly imparted by means of the piston-casing contact when the crank shaft is being rotated.
However, which such a system for generating orbital motion, the machine suffers from the drawback of not being entirely dry, because it requires the presence of gearing to achieve the orbital motion, which gearing must therefore be lubricated to enable long-lasting operation, or else the presence of a pumped lubricating fluid if the gear is omitted and if the orbital motion is directly obtained by means of direct contact between the piston and the casing. In certain applications for which the vacuum must be very clean, this is incompatible.